2. 外语
  3. Toothpaste is a substance【C1】______for the cleaning and polishing the surface of the teeth. Its primary function is【C2】______too

Toothpaste is a substance【C1】______for the cleaning and polishing the surface of the teeth. Its primary function is【C2】______too

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问题    Toothpaste is a substance【C1】______for the cleaning and polishing the surface of the teeth. Its primary function is【C2】______tooth decay. It is only as a by-product of cleaning teeth【C3】______toothpaste bas become a cosmetic.
   Toothpaste【C4】______belongs to a group of dental hygiene products【C5】______as dentifrices. These【C6】______toothpaste, tooth powder, and various bacteria removing mouthwashes.
   Historically, toothpaste is as【C7】______as the first toothache and human beings’first perception that teeth were【C8】______of their attractiveness to the opposite sex. Long【C9】______the invention of the toothbrush, ancient Greeks and Romans scrubbed their teeth with bits of cloth coated with the burnt heads of mice, moles and rabbits in the【C10】______that contact with these strong toothed animals【C11】______make human teeth firm. If these materials【C12】______revolting and dangerous, it is worth thinking【C13】______some other substances that have been used in toothpaste in the past 20 years. After the【C14】______of the toothbrush, it was common to【C15】______toothpaste containing chloroform(a flavoring【C16】______can cause cancer), and lead, which actually poisoned the toothpaste.
   It was not【C17】______the late nineteenth century that teeth cleaning was【C18】______as a vital part of dental health. In 1896, Leon Williams argued that tooth decay was the【C19】______of acid and bacteria, and created the【C20】______"A clean tooth never decays". Although not strictly correct, he did create an environment ill: which dental prevention became the norm.
【C17】
选项 A、after
B、until
C、though
D、since
答案B
解析 not．．．until．．．为固定用法。
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